In the art of electrostatic photocopying or photoprinting, a latent electrostatic image is generally produced by first providing a photoconductive imaging surface with a uniform electrostatic charge, e.g. by exposing the imaging surface to a charge corona. The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light corresponding, e.g., to an optical image of an original to be copied, thereby forming an electrostatic charge pattern on the photoconductive imaging surface, i.e. a latent electrostatic image. Depending on the nature of the photoconductive surface, the latent image may have either a positive charge (e.g. on a selenium photoconductor) or a negative charge (e.g. on a cadmium sulfide photoconductor). The latent electrostatic image can then be developed by applying to it oppositely charged pigmented toner particles, which adhere to the undischarged "print" portions of the photoconductive surface to form a toner image which is subsequently transferred by various techniques to a copy sheet (e.g. paper).
In liquid-developed electrostatic imaging, the toner particles are generally dispersed in an insulating non-polar liquid carrier, generally an aliphatic hydrocarbon fraction, which generally has a high-volume resistivity above 10.sup.9 ohm cm, a dielectric constant below 3.0 and a low vapor pressure (less then 10 torr. at 25.degree. C.). The liquid developer system further comprises so-called charge directors, i.e. compounds capable of imparting to the toner particles an electrical charge of the desired polarity and uniform magnitude so that the particles may be electrophoretically deposited on the photoconductive surface to form a toner image.
In the course of the process, a thin film of the liquid developer is applied to and covers the entire photoconductive imaging surface. The charged toner particles in the liquid developer film migrate to the oppositely-charged areas forming the "print" portions of the latent electrostatic image, thereby forming the toner image and any liquid developer remaining on the photoconductive surface after this stage of the process is recycled back into the liquid developer reservoir.
Charge director molecules play an important role in the above-described developing process in view of their function to control the polarity and charge on the toner particles. Necessarily, counter ions are also created in this process so as to maintain the electrical neutrality of the liquid developer phase as a whole. It is believed that in many liquid developers, the charge director molecules form inverse micelles wherein the polar portions of the charge director molecules are directed inwards to the micelles, while the non-polar portions having the higher affinity to the non-polar liquid carrier, are directed outwards, so as to decrease the overall surface energy of the system. These micelles may solubilise ions generated by the dissociation of the charge director molecules.
The charge director compounds may be classified, in a general manner, into molecular chemical species (hereinafter referred to as "molecular charge directors") and ionic chemical species (hereinafter referred to as "ionic charge directors"). The molecular charge directors are zwitterionic compounds, as exemplified by lecithin, which has proved to be an excellent charge director. The ionic charge directors are mostly metal salts of long-chain organic acids, such as metal soaps or metal salts of sulphonated petroleum hydrocarbons (commercially available under the trade name Metal Petronates).
The choice of a particular charge director for use in a specific liquid developer system, will depend on a comparatively large number of physical characteristics of the charge director compound, inter alia its solubility in the carrier liquid, its chargeability, its high electric field tolerance, its release properties, its time stability, etc. All these characteristics are crucial to achieve high quality imaging, particularly when a large number of impressions are to be produced.
One of the problems encountered in liquid-developed electrostatic imaging is the humidity tolerance of the system, especially at high humidity levels (80-85% relative humidity). It has been observed that some liquid developer systems, when operated in a high humidity environment, suffered from fuzziness of the resulting copies. This problem may be associated with the phenomenon of the so-called "morning sickness", namely that after an electrostatic photocopier machine is left without being operated for a comparatively long period (e.g. overnight or over the weekend), blurry images are obtained and this blurriness or fuzziness persists until after a large number of copies, sometimes a few hundred, are made. Cleaning of the photoconductive imaging surface with solvents, such as isopar, was found to be ineffective in restoring image quality.
The above problems of image fuzziness and morning sickness are believed to be due to the water affinity of the charge director in the liquid developer system. The photoconductive imaging surface is covered by a very thin film of liquid developer containing the charge director. If the charge director tends to solubilise or absorb water, the electric conductance of this layer will increase sufficiently to interfere with the formation of located point charges and allow for lateral conduction, resulting in a fuzzy latent image. The suggested mechanism for the phenomenon of morning sickness is that evaporation of the liquid carrier, (e.g. Isopar) from the aforementioned thin film of liquid developer, leaves a residue containing the charge director on the photoconductive surface and this residue, if hygroscopic, will absorb water from the humid atmosphere, thereby becoming insoluble in isopar. When the electrostatic photocopier is reoperated, the residue does not tend to redissolve easily in the liquid carrier or in other non-polar solvents. The electroconductive residue interferes with the electrostatic imaging process, preventing the formation of localised point charges as explained above.
A typical example of a charge director suffering from the above drawback of sensitivity to humidity, is lecithin which, by most other criteria, is an excellent charge director.
It is accordingly one object of the present invention to provide a charge director material having improved humidity tolerance which, when used in a liquid developer system, is capable of functioning at high hymidity levels without giving rise to the above-mentioned drawbacks of fuzziness and morning sickness.
It is a further object of the invention to provide a process for preparing the above-mentioned improved charge director material.
Another object of the invention is to provide a liquid developer system for use in electrostatic imaging, comprising the above-mentioned improved charge director composition.